The ability to produce refractory filaments of substantial length and useful flexibility is desirable for many applications. For example, non-superconductive fibers can be formed into electrodes for use in electrochemical processes. In a more specialized field, relatively long and flexible refractory filaments of superconductive material can be formed into wires or coils for use in electromagnetic devices.
Unfortunately, such filaments have been extremely difficult to fabricate into wire or tape shapes of the desired strength and flexibility. Previously known ceramic fiber production methods generally are problematic and material specific. Furthermore, production of superconductors is complicated by difficulties in achieving the desired critical current density.
Previously explored methods for making ceramic filaments include the following:
(1) Methods based on certain advanced ceramic forming methods, such as extrusion and tape casting. These methods typically result in a product having low mechanical strength after it is fired;
(2) Extension of the metal-working and diffusion/oxidation techniques originally developed for the low-temperature alloy superconductors, such as NbTi and Nb3Sn. Typical of these processes is the so-called xe2x80x9cpowder-in-tubexe2x80x9d process in which a high-temperature superconductive ceramic powder is packed into a silver tube. The tube is then rolled or drawn into a wire configuration. The wire generally is cracked by the tensile stress either during annealing or subsequent testing due to differences in thermal expansion between the silver sheath and the oxide core;
(3) Organic intermediary methods such as colloidal, sol-gel, and metallo-organic polymer. Each of these methods results in low-density products, due to a very low level of refractory material present in the green body, i.e., prior to heat treatment. Also, colloidal and sol-gel approaches are very slow and hence unlikely to offer commercially viable processes;
(4) Melting-solidification methods, such as melt-textured growth. These have the potential of obtaining dense, oriented crystal wire, but are inherently slow since they are limited by the dynamics of single-point crystal formation and growth;
(5) Alloy oxidation-diffusion methods. These methods have attracted attention because of the good ductility of amorphous alloy wire, and the easy control of the oxygen concentration in the material as its weight changes upon oxidation. However, the process is limited by difficulties in the mixing of immiscible Yxe2x80x94Baxe2x80x94Cu alloy stoichiometrically.
(6) Conventional ceramic melt drawing methods. These are difficult to use due to the incongruent melting of 1-2-3 phase materials. Furthermore, the melting is not practical for high temperature ceramic.
Therefore, a need exists for a process of manufacturing ceramic fiber that provides control and flexibility over the composition and configuration of the fiber. The present invention fulfills this need.
The present invention provides a process of producing fibers of refractory material. In one embodiment, a dispersion of particles of refractory material is prepared first. The dispersion then is mixed with a carrier solution of a salt of cellulose xanthate to form a spin mix. Using general wet spinning techniques, a filament of regenerated cellulose is formed from the spin mix. The filament has the particles dispersed therein. At this point, the filament can be utilized as a mixture of cellulose and refractory material, or it can be heat treated. If heated, the filament is raised to sufficient temperatures and over sufficient durations to remove substantially all of the regenerated cellulose and to sinter the particles of refractory material to form a filament.
The present invention provides a method for producing a wide variety of refractory ceramic filaments, including both conductive and high-temperature superconductive filaments having improved properties.
The present invention also provides a method which is easy and reliable to practice, and which employs known materials and equipment that are readily available in commercial form.
The present invention further provides a method which enables continuous and fast production of high-density grain-oriented filaments with relatively high strength, flexibility, temperature capability and chemical stability. These filaments are especially well suited for use in magnet coils, motor windings, sensors, data transmission wires, electrolmagnetic sheilding, electrodes for use in electrochemical processes, and all other applications in which it is desirable to have refractory material in fibrous form.